A number of general configurations are used for tunable source spectroscopy systems. The lasers have advantages in that very intense tunable optical signals can be generated. A different configuration uses the combination of a broadband source and a tunable passband filter, which generates the narrowband signal that illuminates the sample.
Historically, most tunable lasers were based on solid state gain media. While often powerful, these systems typically have high power consumptions. In contrast, tunable semiconductor laser systems have the advantage of relying on small, efficient, and robust semiconductor sources. One configuration uses semiconductor optical amplifiers (SOAs) and microelectromechanical system (MEMS) Fabry-Perot tunable filters, as described in U.S. Pat. No. 6,339,603, by Flanders, et al., which is incorporated herein by this reference in its entirety.
In commercial examples of the broadband source/tunable filter tunable source configuration, the tunable filter is an acousto-optic tunable filter (AOTF) and the broadband signal is generated by a diode array or tungsten-halogen bulb, for example. More recently, some of the present inventors have proposed a tunable source that combines superluminescent light emitting diodes (SLEDs) and a MEMS Fabry-Perot tunable filter to generate the tunable optical signal. See U.S. patent appl. Ser. No. 10/688,690, filed on Oct. 17, 2003, by Atia, et al., which is incorporated herein by this reference in its entirety. The MEMS device is highly mechanically and spectrally stable and can handle high powers and can further be much smaller and more energy-efficient than typically large and expensive AOTFs. Moreover, the SLEDS can generate very intense broadband optical signals over large bandwidths, having a much greater spectral brightness than tungsten-halogen sources, for example.
One drawback associated with semiconductor spectroscopy systems, however, is limited spectral width. Often a single semiconductor chip is not available that can cover the entire desired scan band.
As a result, a number of tunable semiconductor source have been proposed that are based on using the emission from multiple SLED's or SOA chips in the context of tunable SLED or tunable laser devices.